Members of the Cyathostominae group of nematodes infect almost all grazing horses. Most horses have burdens to the order of tens of thousands of cyathostomins and usually do not exhibit clinical disease, however, in some animals, infection leads to a severe inflammatory enteropathy [15]. This disease occurs following accumulation of cyathostomin larvae that encyst and undergo inhibited development as early third larvae (EL3) in the large intestinal wall. Vast numbers of encysted larvae can accumulate and these can reactivate simultaneously to cause an inflammatory enteropathy known as larval cyathostominosis. The principal effect of this syndrome is weight loss, but horses can exhibit other signs including diarrhea, colic, subcutaneous oedema and/or pyrexia [25]. Up to 50% of animals with larval cyathostominosis die as a result of the condition [15]. This disease most commonly occurs in younger horses, however horses have a lifelong susceptibility to infection and disease may occur at any age [15, 35]. Encysted larvae can persist for prolonged periods (up to two years in some cases) and it has been proposed that encystment is favoured by a variety of factors including; negative feedback from mature worms in the large intestinal lumen, a large larval challenge or a ‘trickle’ infection [29]. cyathostomin EL3 have limited susceptibility to several currently available anthelmintics [12, 19] and drug resistance is common, particularly with regard to benzimidazole and pyrantel compounds [17]. Moxidectin is now only drug available that has high efficacy against EL3, but for which resistance is not yet widespread. It is therefore important that the high efficacy of this anthelmintic be maintained for as long as possible.
To reduce the spread of anthelmintic resistance, it is important that only animals with moderate to high cyathostomin burdens are targeted strategically for treatment [32]. Targeted treatments can be undertaken on the basis of faecal egg counts however the latter have no value in estimating burdens of mucosal larvae. Indeed, horses with high mucosal burdens often have low or negative faecal egg counts [31] and there is no specific, non-invasive method to diagnose pre-patent cyathostomin infection. A diagnostic test for mucosal larvae would allow veterinarians to identify horses that require larvicidal anthelmintic treatments. Recently, we identified two larval antigen complexes (observed to migrate at 20 and 25 kDa by 1-dimensional SDS PAGE) that have diagnostic potential [9-11]. Significant increases in serum IgG(T) specific to these antigen complexes were observed as early as 6 weeks post infection (PI) in experimentally-infected ponies [11]. Antigens present in both complexes appeared to be specific for mucosal larval cyathostomins, indicating their utility as markers of pre-patent infection [11]. When serum IgG(T) levels were compared amongst groups of naturally- and experimentally-infected horses, there was a strong significant correlation of anti-25 kDa serum IgG(T) responses with total mucosal burden, particularly EL3 burden [10]. In naturally infected horses, IgG(T) responses to both larval complexes were significantly greater than those in uninfected individuals [10] and IgG(T) levels to both complexes were significantly higher in larval cyathostominosis clinical cases than in helminth-naïve ponies and parasite-negative horses from an abattoir [10]. These results indicate that an immunoassay based on antigens present in these complexes could ultimately be used to differentially diagnose larval cyathostominosis, or used to target horses with high mucosal burdens for treatment. The native mucosal larval preparations are extremely time-consuming to prepare and rely on a continuous source of infected mucosa. Therefore, it would be advantageous if genes encoding proteins present in these complexes were isolated and cloned and the associated proteins expressed in recombinant form.